1. Field of the Invention
The present invention relates to a method for trimming a thin-film transistor array capable of active-matrix-driving a liquid crystal display element.
2. Description of the Related Art
A thin-film transistor (referred to as "a TFT") array constructed of a 1-pixel and 2-FTT arrangement is widely known in the art where two thin-film transistors are connected parallel to each other for driving a single pixel of a liquid crystal display element.
A typical TFT array constructed of the 1-pixel and 2-FTT arrangement is illustrated in FIG. 4. In this type of TFT array, a large quantity of gate buses 2 and source buses 3 made of molyblenum, or the like are intersected with each other at a right angle on a glass substrate 1. On a crossover portion 4 where these gate buses 2 are intersected with these source buses 3, the source buses 3 run over the gate buses 2, and an insulating film 5 is employed between the corresponding gate bus 2 and source bus 3 not to conduct these intersected buses 2 and 3. As illustrated in FIG. 4, one gate line 6 is branched from the gate bus 2 adjacent to each of the crossover portion 4, whereas two source lines 7 and 7 are similarly branched from the source bus 2. These branched gate line 6 and source lines 7, 7 are connected to a common gate electrode and two source electrode of a pair of TFT 8 respectively, which are formed adjacent to the crossover portion 4. This TFT (thin-film transistor) 8 is constructed of a first TFT 8a and a second TFT 8b, which are connected parallel to the source bus 3, gate bus 2 and pixel electrode 9. Either the first TFT 8a or the second TFT 8b drives one of the liquid crystal display element 1.
In such a conventional TFT array, a shortcircuit defect may occur unavoidably in the manufacturing process of the TFT array, which is caused by the various reasons. As the typical shortcircuit defect, the gate electrode is shortcircuited with the source. In the TFT array constructed of 1-pixel-2-TFT arrangement, as illustrated in FIG. 4, the shortcircuit, in all probability, is produced in the positions indicated by (A), (B) and (C) in FIG. 4. Since such a shortcircuit defect appears as the line defect or point defect in the liquid crystal element, the shortcircuit defect is cut out from the gate electrode or source by means of the laser trimming method in the conventional TFT array manufacturing process.
According to one of the trimming process for the shortcircuit defect occurring at any point of (A), (B) and (C), at first, an intermediate position .circle.1 of the source bus 3 between the branch points of two source lines 7 and 7 thereof is cut out. Thereafter, the source bus 3 is cut by way of the trimming process at a position indicated by .circle.2 between the crossover portion 4 of the source bus 3 and the branch point of the source line 11. As a result, if the shortcircuit has been made at a point (A) of the first TFT 8a, this shortcircuit defect is cut out from the source bus 3. In addition, if the source bus 3 is cut out at a position .circle.3 adjacent to a terminal S1 of the crossover portion 4 of the source bus 3, the shortcircuit occurring at a point (B) of the crossover portion 4 is cut out from the source bus 3. Then, when the source bus 3 is cut at a position .circle.4 adjacent to another terminal S2 of the source line branch point of the source bus 3, the shortcircuit which has occurred at the point (C) of the second TFT 8b is cut out. As a consequence, even if the shortcircuit occurs at any points (A), (B) and (C) of the source bus 3, the shortcuited source bus portion can be cut out from the source bus 3. Then, if power is supplied via both terminals S1 and S2 of the source bus 3, one of the first and second TFT 8a and 8b can be brought into the active condition, resulting in prevention of the point defect.
However, the above-described laser trimming method involves the following drawback, where the laser trimming is successively performed from the position .circle.1 to the remaining positions .circle.2 3 and .circle.4 in this order. That is to say, if the shortcircuit defect occurs only in the point (B) of the crossover portion 4, the first TFT 8a is necessarily cut out in which no shortcircuit problem is present, i.e., no problem with the first TFT 8a. Accordingly, since the second TFT 8b can be normally operated, there is no point defect. However, the resultant contrast ratio of the liquid crystal element which is driven by solely the second TFT 8b, is lowered, as compared with that by both the first and second TFT 8a and TFT 8b.
The present invention has been accomplished in an attempt to solve the problems mentioned above, and has an object to provide a novel trimming method by which the normal TFT is not cut out, in the laser trimming method for the TFT array of one-pixel-2-TFT circuit arrangement.